The invention concerns a device designed for attaching an aircraft engine to a pylon secured to a structural aircraft section such as the wing or fuselage.
More precisely, the invention concerns an attachment device designed to transmit to the aircraft structure, via the engine pylon, the loads acting on the engine in the lateral and vertical directions relative to the longitudinal axis of the engine, for the case where the engine is suspended under the wing. When the engine is mounted on the side of the fuselage, the attachment device concerned by the invention, is designed to transmit the loads applied in the lateral and vertical directions in relation to the longitudinal axis of the said engine.
In the remainder of the text, only the case of an engine suspended beneath an aircraft wing is described. The description below nevertheless applies also to the case of an engine mounted on the side of the fuselage, by replacing the loads acting in the vertical direction with loads acting in the horizontal direction.
The engines fitted to aircraft are suspended beneath the wing or mounted laterally on the fuselage via a device known as the xe2x80x9cengine pylonxe2x80x9d. For simplification, this device is hereafter referred to as the xe2x80x9cpylonxe2x80x9d.
The link between the engine and the pylon is generally provided by two or three complementary attachment devices, each performing distinct functions.
One of these attachment devices is intended to take up the engine torque as well as the loads generated by the engine in the lateral and vertical directions, in order to retransmit them to the aircraft structure via the pylon. The invention precisely concerns an attachment device of this type. It should be noted that such a device may be fitted indifferently at the front or rear of the engine, the dimensions of the constituent parts being adjusted in consequence.
As illustrated in particular by documents U.S. Pat. Nos. 5,620,154 and 5,275,357, certain of the existing attachment devices include a main attachment structure and a backup attachment structure. The main attachment structure transmits the loads in normal operation, i.e. when all the parts of the attachment device are intact. The standby attachment structure is intended to provide continuity of the load transmission function between the engine and the aircraft in the event of failure of one of the parts of the main attachment structure.
The existing attachment devices, such as those illustrated in the documents listed above, habitually include an intermediate bracket, attached to the engine pylon for example via bolts, and at least two spherical bearing equipped rods, which provide the link between the bracket and the engine. The articulated attachment of the rods to the bracket and to the engine prevents the structure from being subjected to loads along the longitudinal axis of the engine. This arrangement also compensates for the thermal expansion of the engine and preserves the isostatic nature of the link between the pylon and the engine.
As is also illustrated by documents U.S. Pat. Nos. 5,620,154 and 5,275,357, when the existing attachment devices include standby attachment devices, the latter generally include additional items, such as one or more additional rods inserted between the bracket and the engine. These additional items incorporate play such that they do not normally take up any load when the constituent parts of the main attachment structure are intact and, in consequence, are able to perform their functions.
Certain existing attachment devices, such as those used on xe2x80x9cAIRBUSxe2x80x9d aircraft, include an intermediate part inserted between the pylon and the bracket. This intermediate part has a trapezoidal section when viewed in cross-section in a plane passing through the longitudinal axis of the engine. It exhibits a first flat surface intended to be attached, for example via four bolts, to a flat inclined attachment surface on the pylon and a second flat horizontal surface for attachment of the bracket, for example via four other bolts. The intermediate part may be in one piece or made up of two half-parts assembled together by bolts.
In the existing attachment devices, such as those described in documents U.S. Pat. Nos. 5,620,154 and 5,275,357, the additional parts which constitute the backup attachment structure are invariably inserted between the main attachment structure bracket and the engine. In other words, these additional parts preserve the integrity of the engine attachment when the link between the bracket and the engine is jeopardised. However, they are not able to preserve this integrity if the failure concerns the link between the pylon and the bracket, i.e. for example the attachment bolts connecting these two items or the intermediate part, if the latter exists.
Moreover, the combination of main and standby attachment functions through separate additional items leads to attachment devices which are both heavier and more complex. This goes against the permanent objectives in the aeronautical field, which are weight saving, simplification and reliability.
Additionally, when the loads to be transmitted are very large, in particular in the case of heavy high-thrust engines, it may be found necessary to reinforce the attachment device. This then leads to an increase in the number of attachment points. Thus, the attachment of the bracket to the intermediate part, when it exists, or directly to the pylon must be provided by two rows of four bolts instead of two rows of two bolts as used habitually. This means that the bracket attached to the pylon must be made stiffer, which is achieved by increasing its dimensions. Thus, the width of the bracket may, for example, be virtually doubled relative to that of a conventional bracket.
With such an arrangement, it is easy to see that the loading of the four bolts is not uniform. Indeed, the two bolts located outboard transmit little load relative to the two bolts located inboard. In fact, about 20% of the loads are applied to the outer bolts and 80% of the loads are applied to the inner bolts. With time, this proportion generates much greater structural fatigue of the inner bolts than of the outer bolts. This leads to a much higher probability of failure for the inner bolts.
This problem could be resolved by reinforcing the inner bolts. However this would involve resorting to a specific tool for tightening these bolts. Additionally, such bolts would protrude into the airflow and generate drag. Also, in view of the limited space available in the attachment device, this solution is difficult to apply in practice.
The invention precisely covers a device for attachment of an engine to a pylon on an aircraft, whose original design preserves the integrity of the attachment of the engine even in the event of failure of one of the parts of the main attachment structure or of the intermediate part, if one exists, whilst at the same time providing homogeneous distribution and transmission of the engine loads to the pylon.
In accordance with the invention, this result is obtained by using an engine to aircraft pylon attachment device, consisting of two assemblies forming lateral rods, and means of linking able to separately connect each of the assemblies forming lateral rods to each of the items constituted by the pylon and the engine, characterized in that each of the linking means includes a pin passing through the said rods and the central web of a female yoke without play, and passing through the side webs of the said female yoke with play.
According to a preferred production method for the invention, the attachment device additionally includes an assembly forming a bracket, and means of attachment by which the assembly forming a bracket is able to be attached to the pylon, and in which the assembly forming a bracket includes at least two brackets and each assembly forming a lateral rod includes two rods able to separately connect each of the brackets to the engine.
The presence of an assembly forming a bracket and the duplication of the bracket and rods to take up the loads between the engine and the brackets, therefore enable the deletion of a standby structure such as described in documents U.S. Pat. Nos. 5,620,154 and 5,275,357. Indeed the attachment device in accordance with the invention is capable of taking up the loads in the various possible failure cases, without the need for any additional parts.
Additionally, whereas the attachment devices described in the two above-mentioned documents do not preserve the integrity of the engine attachment in the event of a failure of the link between the pylon and the bracket, the attachment device in accordance with the invention also covers this type of failure.
In other words, the attachment device in accordance with the invention is able to preserve the total integrity of the link between the pylon and the engine, whilst remaining simple and reliable.
According to another preferred production method, although not restrictive, of the invention, the attachment device additionally includes an assembly forming an intermediate part, inserted between the two brackets and the pylon. This assembly forming an intermediate part consists of two intermediate parts and the means of attachment include the first means of attachment of each bracket to one of the intermediate parts and second means of attachment of each intermediate part to the pylon.
Moreover, it should be noted that the duplication of the rods taking up the loads and their attachment to the bracket with bolts provide enhanced distribution and reliable transmission of the loads in the said bolts, the load applied to each bolt being equal to the same fraction of the total load to be transmitted.
The means of linking each assembly forming a lateral rod to the brackets preferably include two female yokes belonging to each of the brackets, and whose overlapping webs form the said central web.
Additionally, spherical bearings are advantageously inserted between the pin on the one hand and the rods and the central web of the female yokes on the other hand. This arrangement permits to give greater mobility to the pin. In some extreme conditions, the pin can contact the edge of the bore provided within the lateral webs of the female yoke. Then, the loads can be transmitted via one or the other of the lateral webs of the female yoke. As the loads are extreme, it is thus easier to distribute it in such conditions. The gap between the pin and the bores provided in the lateral webs must be sufficient to prevent the pin from any contact with the edge of the bores under normal load conditions, in order to avoid unnecessary fatigue of the lateral webs.
Then, the device acts simultaneously as a main attachment structure and an emergency attachment structure and, in the event of extreme loads, it provides an additional path for transmitting the loads from the engine to the pylon.